X0303 4 2009-02-06 0001097864 ON SEMICONDUCTOR CORP ONNN 0001368095 Mahoney Robert Charles 5005 EAST MCDOWELL ROAD PHOENIX AZ 85008 1 Exec VP Sales & Marketing Common 2009-02-06 4 S 0 24767 4.885 D 69999 D This disposition transaction was executed in multiple trades at prices ranging from $4.82 to $4.95. The price reported above reflects the weighted average sale price. The Reporting Person hereby undertakes to provide upon request to the SEC staff, the Issuer or a security holder of the Issuer full information regarding the number of shares and prices at which the transaction was effected. These shares were initially acquired pursuant to a performance based restricted stock units award made to the Reporting Person by the Issuer's Board of Directors/Compensation Committee effective June 2, 2008 under the Issuer's 2000 Stock Incentive Plan with vesting tied to the achievement of specified performance metrics, subject to the plan and relevant award agreement. Judith A. Boyle is signing on behalf of the Reporting Person pursuant to a Power of Attorney attached herein as an exhibit. mahoney2008poa.TXT Judith A. Boyle, Attorney-in-Fact 2009-02-10